1. Field of the Invention
The present invention relates to a silicon nitride film forming apparatus and forming method having excellent reproducibility of film thickness and film quality, and in particular to a silicon nitride film forming apparatus and forming method used for compound semiconductor devices such as HEMT (High Electron Mobility Transistor), HBT (Heterojunction Bipolar Transistor) and the like.
2. Related Art
Chemical vapor deposition methods, such as a thermal CVD method and a plasma CVD method, have been widely used for manufacturing semiconductor devices. However, as the higher integration and function of devices come to be requested, the damage caused by the plasma and processing temperature become serious problems. Under such circumstances, a Catalytic CVD (Cat-CVD) method, which is free of such problems and can form a high quality film, is paid much attention and studied as a practical film forming method for a variety of thin films (JP10-83988A).
This method is carried out using an apparatus in which a substrate holder 3 holding a substrate 7 and a heating element 4 made of refractory metal such as tungsten or the like are disposed facing each other in a vacuum vessel 1 having an exhaust port 2 and a gas introduction means 8, as shown in, for example, FIG. 3. First, the inside of the vacuum vessel is exhausted. The heating element is heated by supplying electric current from an external power source through a connection terminal 5, and maintained at a temperature of from about 1000° C. to 2000° C. Then, a raw material gas is introduced into the vacuum vessel from gas introduction means 8 and decomposed and/or activated on the surface of heating element and/or in the vicinity to begin the deposition of thin film on the substrate.
It was confirmed by applying this method to the formation of the silicon nitride passivation layer of, for example, a GaAs HEMT that the damage on substrate surface is reduced and the device manufactured has excellent dynamic characteristics (R. Hattori et al., Technical Digest of 19th Annual GaAs IC Symp., Anaheim, 1997, p. 78).
However, when the silicon nitride film was repeatedly formed on a number of substrates, the film quality and film thickness were found to vary from substrate to substrate, meaning that the device characteristic was not stably obtained.
Then, in order to clear up the cause, the present inventors have investigated the structure and arrangement of members in the apparatus, the conditions of film formation and pretreatment such as outgas treatment, the cleaning to remove films deposited on the inner surface of vacuum vessel and the like, and, as a result, found that the reproducibility of film quality and film thickness remarkably fluctuates, depending on the temperature and the configuration of a deposition shield plate which was placed to prevent the film deposition on the inner surface of vacuum vessel and make the maintenance easier. The deposition shield plate is generally connected to the vacuum vessel directly or indirectly in such apparatuses, and its temperature changes with the temperature of heating element. That is, the temperature rise and drop of deposition shield plate, together with vacuum vessel, repeatedly take place, corresponding to the start and stop of film formation. Therefore, the relation of the film quality and film thickness with the temperature of deposition shield plate was examined carefully to find that the reproducibility of film quality, film thickness and the like sharply changes with the temperature around the heating element and the substrate.